Tuesday, January 29, 2013

Scientists' Dueling Letters to the Editor Re: Warming

In dueling letters in the Oregonian, scientist David Douglass of the University of Rochester responds to Andreas Schmittner's letter to Gordon Fulks op-ed:

Schmittner is wrong when he says that the statement "global temperatures have not risen during the past 15 years" is not true. Most climate scientists agree that that the earth has not warmed during the past 15 years.

Douglass is simply wrong -- the ocean has warmed strongly in that time, according to NOAA data:

This means the top half of the ocean has warmed by about 0.8 × 1022 J/yr, or an average of 250 terawatts (TW).
Divided over the entire Earth's surface (viz. assuming all the extra heat goes into the ocean) it comes to an average of 0.5 W/m2.
That's a lot of warming.

The two-dimensional surface of a planet with a thin gas above it is about the worst place to look for the kind of energy imbalance that additional greenhouse gases create. The oceans have a heat capacity (mass × specific heat) about 1,000 times that of the atmosphere.

Think of it this way: Imagine you have a children's swimming pool with a strong light shining down on it, with a rock in the middle of the pool that sticks above the surface.

Now you want to know, is the light above getting more intense?

Where are you going to look? Are you going to set your thermometer a few millimeters just above the surface of the rock, and see if the temperature there is changing?

Or are you going to stick your thermometer in the water and see if that's changing.

You will, of course, stick it in the water. The rock is close to the water so swirls in the water near the rock can affect the temperature you measure there, and winds carry heat to and from the oceans, etc.
No one would dream of measuring just above the rock (though you'd surely start there, because you don't want to get your feet wet).

Of course, we live on such a rock, and that's ultimately the temperature we care about. But anyone would certainly expect, in the children's swimming pool, that if you measure the water warming up, the thin layer of air just above the rock is eventually going to warm up too.

It's no different. The surface is subject to natural variations, and choosing 15 years is a cherry pick. (Why not choose 12? Or 18? Or 30?)

Not to mention, there has been surface warming in the last 15 years. It's just that it hasn't risen above its statistical uncertainty by the canonical 2-sigma.

HadCRUT4's 15-yr trend is, from the SkS trend calculation, 0.098 ± 0.143 °C/decade (2σ). That's a positive trend, i.e warming, and statistically significant at the 83% confidence level. But note that it would have to be above 0.14 °C/decade to be statistically significant at the 95% confidence level, and that's not always going to be the case for short time intervals. Sometimes it will be even stronger, like the period 1975-1998. But picking that period would be cherry picking, too.

Unless temperature increases monotonically upward, with, say, each month 0.001 °C warmer than the month before, contrarians will always be able to find some period where warming is less than 2σ (95%) significant. If it's not 15 years it will be 12 years or 7 years or something. So this game is about all they have left to rely on.

And you can bet than when the next El Nino comes and surface temperatures shoot up, they will be crying that the increased trend isn't a proper measure because it cherry picks the last few years. Bet on it.

3 comments:

Schmittner is an actual climate scientist. Douglass is a physicist who plays a climate scientist in physics journals. Most of his climate-related research deals with "climate shifts", which is basically an effort to claim that every little bit of short-term noise in the climate system is indicative of some sort of "shift".

As we showed in Nuccitelli et al. (2012), the data Douglass has used in his research does show that global warming continues.

What bugs me about such statements as "the earth has not warmed during the past 15 years" is that it is a complete misrepresentation of statistics. It is accurate to say that statistically, the global temperature data are consistent with no increase in global temperature. However, it is just as accurate to say that the global temperature data are consistent with a warming trend of 0.2 degC per decade. Both are within the 95% confidence range. The problem is that the staement that no warming has occured is exactly equivalent to the statement that warming of at least 0.2 degC per decade has occured. Clearly both staements can't be true so it is not appropriate to make either claim!